MOSCOW (Reuters) - Hundreds of riot police deployed in central Moscow on Saturday ahead of an opposition protest calling for President Vladimir Putin not to run for what would be his fourth presidential term next year.

Putin, who has dominated Russia's political landscape for the last 17 years, has not said whether he will run in March 2018, but official pollsters give him high popularity ratings and he is widely expected to do so.

The pro-democracy Open Russia movement, founded by Kremlin critic Mikhail Khodorkovsky, called on people to converge on the presidential administration in Moscow on Saturday afternoon to hand over written appeals for Putin to leave political life.

It was seeking to organize similar protest events in other cities too, including St Petersburg.

Under the slogan "We're sick of him", Open Russia is seeking to build on what the liberal opposition regarded as the success of a wave of anti-government protests last month whose geographical reach and turnout were the largest since 2012.

The authorities stepped up pressure on Open Russia ahead of Saturday with the Kremlin saying the planned protest was illegal and would be dealt with accordingly by the police.

The General Prosecutor's Office ruled on Wednesday that the activity of Open Russia's British arm was "undesirable" in Russia, accusing it and other organizations of trying to stir up trouble aimed at discrediting the presidential election.

Police on Thursday searched the Moscow offices of Open Russia's Russian branch and confiscated 100,000 blank appeal forms which the foundation had hoped to hand out to people encouraging them to call for Putin to quit.

And on Friday, REN TV, a Russian TV channel, broadcast a documentary about Open Russia activists, some of whom it accused of having criminal records, of being drug addicts, and of cultivating close links with the U.S. government.

Repeatedly showing images of Khodorkovsky, U.S. Republican Senator John McCain and U.S. financier George Soros, it suggested opposition activists were motivated by money and getting funding from shadowy foreign sponsors.

Activists dismissed the program as a cheap stunt designed to discredit them, with at least one noting that REN TV had somehow obtained video footage stored in his mobile phone.